The present invention relates to a multi-pass rolling method which can permit a single rolling-mill stand to perform multi-path rolling and can stabilize the rolling operation by preventing the lateral deviation movement of a metal workpiece being rolled, thereby producing rolled products having a satisfactory degree of shape or flatness and to a multi-pass rolling-mill stand best adapted for carrying out said multi-pass rolling method.
In rolling operation, under some rolling conditions, a workpiece being rolled cannot remain at a predetermined pass between a pair of upper and lower rolls so that, as indicated by the one-dot chain lines in FIG. 1, the workpiece being rolled is caused to displace itself toward one ends of the pair of upper and lower rolls. This phenomenon, the parallel displacement to the width direction of the workpiece is well known in the art and is called "lateral deviation movement" in the specification.
The lateral deviation movement of a workpiece being rolled by a conventional rolling-mill stand will be briefly described below. In the case of rolling a workpiece by a rolling-mill stand, the rolling pressure exerted at the work side (the side away from drive means for driving rolls) becomes different from that exerted at the drive side (the side near the drive means) due to the rolling operation conditions such as the difference in hardness in the widthwise direction of a workpiece being rolled, a taper in the widthwise direction of a workpiece being rolled, the off-center (that is, the centerline of a workpiece being rolled being not correctly aligned with the center of a pair of upper and lower rolls) and so on, so that there results a difference in roll gap between the work and drive sides. Therefore there occurs difference in elongation of a workpiece between the work and drive sides and the entering velocity of the workpiece becomes faster on the side at which the gap is increased. As a result, as shown in FIG. 1, the upstream portion of a workpiece a being rolled is inclined as indicated by an arrow e relative to the rolling direction (indicated by an arrow c) and the inclined workpiece a being rolled is drawn in the direction perpendicular to the axis of the roll b so that the workpiece a being rolled is caused to be displaced laterally in the direction in which the roll gap is increased. Therefore the difference in roll gap between the work and drive sides is further increased. In this case, a roll gap as shown in FIG. 2 is formed and the lateral deviation movement of a workpiece a being rolled results in the direction indicated by an arrow f. If the lateral deviation movement of the workpiece a being rolled occurs, the workpiece a being rolled does not return by itself to its normal path; some countermeasure is needed to return it in position.
The lateral deviation movement in question is a utterly unstable phenomenon from the viewpoint of control engineering. Once happening, it cannot be suppressed without using some positive control means, as mentioned above. This is further explained with respect to FIGS. 17(A) to 17(D). Little asymmetry causes slight roll skewing (FIG. 17(A)); and the strip a is drawn in faster at the wider roll gap side (FIG. 17(B)) so that the strip a is inclined as shown in arrow e against the direction c of the travel (FIG. 17(C)). As a result, the strip a goes away from the center of the track ever faster (FIG. 17(D)). Therefore the difference in roll gap between the work and drive sides is further increased. These steps repeat themselves so that the lateral deviation develops more and more, resulting in further and further deviation of the strip towards one side.
As described above, when the difference in roll gap between the work and drive sides (to be referred to hereinafter as "the left and right sides", respectively) of the rolling-mill stand, the lateral deviation movement of the workpiece being rolled results. It follows therefore that in order to prevent such lateral deviation movement, the roll gap on the side of the displacement of the workpiece being rolled must be decreased, which is clear from the above-mentioned analysis on the lateral deviation movement.
Meanwhile RD (Rolling Drawing) process in which a workpiece being rolled is wrapped around work rolls having different peripheral velocities has been proposed and demonstrated so that the rolling-mill stands can be made compact in size, roll wear is minimized, it becomes possible to roll hard metals such as high tension steel and edge drops are reduced. As one of the modifications of the RD process, there has been devised and demonstrated a one-stand multi-pass rolling process in which three or more work rolls having different peripheral velocities are arranged one above the other and a workpiece being rolled is wrapped around the work rolls so that the workpiece is rolled at each pass between the adjacent work rolls. There has been also proposed and demonstrated another modification also called a one-stand multi-pass rolling process in which a workpiece being rolled is not wrapped around the work rolls, but is passed between the adjacent work rolls. As compared with the one-stand single-pass rolling process, the one-stand multi-pass rolling process can roll a workpiece under a high reduction at a relatively low rolling force and has a high degree of productivity. In addition, the one-stand multi-pass rolling process is adapted to make a rolling line compact in size.
However, in both of the one-stand multi-pass rolling process and the one-stand single-pass rolling methods, when there occurs the difference in roll gap between the work and drive sides of the rolling-mill stand, the lateral deviation movement of a workpiece being rolled results. Once the lateral deviation movement occurs, it is very difficult to return the workpiece being rolled to a predetermined stable path. Furthermore, the difference in rolling gap causes the incorrect shape of a final product.
In order to prevent the lateral deviation movement in the one-stand multi-pass rolling-mill stand, it may be proposed to apply tension to a workpiece being rolled at the entry side of the rolling-mill stand; but in the case of upper stream of the cold rolling, a considerably great power is required to apply a tensile force to a workpiece because of its thickness. For instance, if the non-parallelism between a workpiece to be rolled and the work rolls is 30 .mu.m in the first rolling pass, a back tension on the order of 3 kg/mm.sup.2 must be applied. If a metal workpiece is 4 mm in thickness and 1000 mm in width and the entering velocity is 500 m/min, the motor power as much as 1000 kw would be needed.
In view of the above, one of the objects of the present invention is to easily and positively prevent the lateral deviation movement of a workpiece being rolled without the need of a high power to substantially eliminate the damages at the edges of the workpiece being rolled and to prevent the workpiece from being broken or cracked and to ensure the stabilized rolling, thereby improving the rolling efficiency and the yield and providing rolled products having a satisfactory degree of shape or flatness.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of preferred embodiments thereof taken in conjunction with the accompanying drawings.